Using shared private network

Number of tempest test cases search for shared private networks
and use value of fixed_network_name parameter from tempest.conf
file in case when multiple shared networks are found.

Private network created by functest is updated to be "shared".
Added routine for network updating into functest_utils.py. Note
that currently used OpenDaylight release (Lithium) doesn't
support shared private networks so network updating will fail
(this is however not a fatal error).

Change-Id: I5a9a0f950b3ef61b6ebfda1af4345460134cc0e3
Signed-off-by: Viktor Tikkanen <viktor.tikkanen@nokia.com>